Audio amplifying and transducing apparatus for producing and emitting a high-amplitude high-energy sound output



Apnl 8, 1969 w. R. STEWART 3,438,028

AUDIO AMPLIPYING AND TRANSUUCING APPARATUS FOR PRODUCING AND EMITTING AHIGH-AMPLITUDE, HIGH-ENERGY SOUND OUTPUT Sheet of 2 Filed March 10, 1966I AUTOMATIC i "1 CYCLING LME-lit INVENTOR. 3 WALTER R.STEWART FIG.2

April 8, 1969 w. R. STEWART 3,438,028 AUDIO AMPLIFYING AND TRANSDUCINGAPPARATUS FOR PRODUCING AND EMITTING A HIGBFAMPLITUDE, HIGH-ENERGY SOUNDOUTPUT Sheet Z of2 Filed March 10, 1966 FIG. 5

FIG. 4

INVENTOR.

WALTER R. STEWART Us or. 340-384 United States Patent 3,438,028 AUDIOAMPLIFYING AND TRANSDUCING APPARATUS FOR PRODUCING AND EMIT- TING AHIGH-AMPLITUDE HIGH-ENERGY SOUND OUTPUT Walter R. Stewart, 8264 E.Beverly Drive, San Gabriel, Calif. 91775 Filed Mar. 10, 1966, Ser. No.533,342 Int. Cl. G081) 3/10 9 Claims ABSTRACT OF THE DISCLOSURE Thespecification discloses an audio amplifying and transducing apparatusfor producing and emitting a highamplitude, high-energy sound outputwhich may comprise a siren type of signal and/ or an oral or vocal typesignal and wherein the means for producing the siren type signal iscompletely self-contained and the means for producing an oral or vocaltype signal is completely self-contained except for an audio inputthereto so that it may be said that in this mode the apparatus comprisesa public address signal producing and amplifying system. The dualpurpose apparatus is embodied in an extremely small, compact, ruggedunit adapted to be mounted on an emergency vehicle and to functionefiiciently during the rather severe environmental conditions to whichit will be subjected while so mounted.

Generally speaking, the present invention relates to an audio amplifyingand sound producing system which, in one of its preferred forms, maycomprise a siren signal producing and amplifier system and/or a publicaddress signal producing and amplifying system, both embodied in asmall, compact unit adapted to be mounted on an emergency vehicle, suchas a police motorcycle, police car, an ambulance, a fire engine, arescue unit vehicle, a civil defense vehicle, a military vehicle, or thelike, and which is capable of producing a very high-powered andhigh-amplitude acoustic output of sufiicient magnitude to comprise anoutput sound of an undulating siren type, a steady-state siren type, ora greatly amplified voice in the public address mode of operation of theapparatus, which is adapted to be publicly acoustically broadcast andeasily heard by persons over a wide special area.

It should be noted that, while the apparatus of the present inventionhas been referred to hereinbefore as being particularly adapted to bemounted on an emergency vehicle such as a police motorcycle, police car,ambulance, fire engine, rescue unit vehicle, civil defense vehicle,military vehicle, or the like, it should be clearly understood that theinvention is not specifically so limited and,

indeed, may be carried by virtually any type of vehicle Whether of aground-borne type, a water-borne type, or an airborne type such as ahelicopter, for example. Also, the apparatus may be employed in thepublic address mode of operation thereof wherever a public addresssystem would be desirable, such as in a ball park, sports arena, or thelike, for example.

In certain forms of the invention, the siren signal producing portion ofthe apparatus may be employed for producing conventional siren outputsound signals of the undulating type normally employed by emergencyvehicles. This form and application of the invention is particularlysuitable for police motorcycles, and the like. However, the siren signalproducing portion of the apparatus may also be employed for producing anonundulating siren type output sound, where such is desired. Additionalreference to this will be made hereinafter.

Also, as pointed out above, the apparatus may be em- Patented Apr. 8,1969 ployed as a very effective public address system for amplifying thehuman voice, or the like, where such is desired. This is most oftenemployed in police cars, and other emergency cars, or the like. However,it should be noted that any of the subcombination, or completecombination, forms and modes of application of the present invention maybe employed on virtually any type of vehicle where it is desired, orelsewhere, and all such arrangements are intended to be included andcomprehended Within the broad scope of the present invention.

The siren signal producing portion of the apparatus is provided with asiren signal generator means and controllably operable siren activationmeans for effectively causing said siren signal generator means to be inoperative input relationship with respect to an audio output amplifierportion of the apparatus. Said siren activation means may becontrollably manually operated in a manner such as to cause anundulating siren output sound or may be provided with automatic cyclingmeans for causing such undulations in the siren output sound at adesired ifrequency and/or for a desired length of time if automatictermination is desired after a certain elapsed time.

This may be accomplished by any conventional cycling apparatus, such asa clock-operated cycling switch, various types of cycling relays, or thecycling means more particularly disclosed and claimed in my co-pendingpatent application Ser. No. 477,901, filed Aug. 6, 1965, and referenceis made thereto for the details of one representative, butnonspecifically limiting, type of such a cycling apparatus which may beused for this purpose.

Also, the timer or timing means disclosed in my said copendingapplication may be employed for causing an automatic termination of theoperation of the siren signal producing portion of the apparatus,whether in the undulating or steady-state mode of operation, after apredetermined period of time. However, it should be understood that saidtimer or timing means disclosed in my said co-pending patent applicationis exemplary only of many other such timers which may be employed forthis purpose within the broad scope of the present invention.

It should be noted that various proposals have been made for providingproper warning to the public of both civil and military emergencies,which may include civil and military disasters and military attacks. Incertain cases, these proposals have specified that a siren type outputsound of the undulating type be employed and should terminate after aspecified period of time, such as three to five minutes and, in certainother cases, these proposals have specified that a siren type outputsound of a steady state or undulating type be employed and automaticallyterminate after a predetermined period of time such as three to fiveminutes.

Either of the above-mentioned proposed emergency or attack and disasterWarning signals may be readily produced by the siren signal producingportion of the apparatus of the present invention by merely controllingthe siren activation means thereof by the above-mentioned cycling meansand/or timing means disclosed in detail in my above-identified copendingpatent application and, therefore, it is clearly apparent that theapparatus of the present invention is widely adaptable to virtually alltypes of emergency vehicle broadcasting purposes, whether for thepurpose of producing and broadcasting a conventional emergency vehicleundulating type output siren sound, or either an undulating,automatically terminating, or a steady state, automatically terminatingcivil disaster or military disaster warning siren type output sound, orfor producing a greatly amplified voice-type output sound when operatedin the public address mode, and this is accomplished in a very small,compact, relatively foolproof, and yet extremely high-powered type ofamplifying system by reason of the novel constructional featuresemployed therein.

It should be noted that the present invention and the presentapplication are directed to the complete combination form of theinvention and subcombination thereof as referred to hereinbefore andthat the novel amplifier means and/or the novel preamplifier means maybe collectively or individually used, per se, for a variety of purposesother than those specifically set forth in detail herein, and all sucharrangements are intended to be included and comprehended within thebroad scope of the present invention.

It should be noted that the transistors and diodes employed in apreferred form of the invention are of an all-silicon construction, thatno coupling capacitors are required, and that there is zero stand-bycurrent for the siren unit, thus making it unnecessary to provide anyon-and-off switch.

It should further be noted that in a preferred form of the invention, nosudden cut-off of siren sound output at the low frequency end of thesiren wail cycle occurs because of the provision of novel siren Wailcut-off retarding means, which acts to produce a frequency fall rateefiectively prolonged so as to simulate that produced by a conventionalmechanical air siren.

It should also be noted that, in one preferred form of the invention, aquick-acting electromagnetic siren brake is provided for the purpose ofquickly stopping the siren when desired.

Siren wail may also be terminated abruptly by deactivating thepreviously mentioned audio output amplifier portion of the apparatus towhich the siren signal generator is coupled, or this may be accomplishedby interrupting the coupling of the siren signal generator with respectto said audio output amplifier portion of the apparatus. It is notpossible to provide such an abrupt termination of a siren output signalin a conventional air siren.

Another feature of the invention is the ability to use the publicaddress mode of operation of the apparatus, or the siren signal mode ofoperation of the apparatus, at any time without the use of a changeoverswitch. Activation of a microphone switch only is suflicient foreffectively placing the apparatus in condition for operation in thepublic address mode of operation, producing only a small, almostnegligible current drain from a battery adapted to supply power to theapparatus until desired audio information appears at the microphone.

Even when the apparatus is still in the siren signal mode of operation,closure of the microphone switch will automatically withdraw thepreviously mentioned audio output amplifier from a square-wave saturatedsignal condition, and the siren tone will become a subdued backgroundsine wave sound, capable of being easily overridden by the publicaddress signal produced by an acoustic input into the microphone.

A preferred form of the apparatus employs a multiplestage,direct-coupled, push-pull audio output amplifier means, which, when thesiren activation means is operating, is directly coupled thereto andbias is applied to said audio output amplifier means whereby to cause itto operate as a class C amplifier means which is fully protected againstdissipation due to overdriving.

The audio output amplifier means is coupled to a pushpull audio outputtransformer means which, in one preferred form of the invention,comprise a tape-wound highly-efiicient autotransformer which drives anoutput electroacoustic transducing loudspeaker at the appropriatevoltage for desired power output.

All transistors in said multiple stage audio output amplifier means aredriven fully into saturation except when operated as a class B amplifiermeans in the public address system mode of operation to be describedhereinafter whereby to minimize power dissipation within thetransistors.

The public address system preamplifier, in one preferred form of theinvention, is a two-stage, direct-coupled, phase-splitting preamplifiermeans provided with a unique apparatus and method for automaticallyproducing the proper biases and equal out-of-phase signals at the inputterminals of the hereinbefore-mentioned multiple stage audio outputtransformer means without the use of coupling capacitors. The bias whichappears when the public address system activation switch (the microphoneswitch) is closed, changes the apparatus to class B operation for goodvoice reproduction.

Output sound volume from the loudspeaker means is controlled by a novelDC balanced negative feedback circuit, without frequency discrimination.This allows maximum possible inverse feedback when volume is set at aminimum value so that reproduction fidelity at low volume is of veryhigh quality. Volume increases as the negative feedback is controllablygradually reduced until a mechanical stop on the control sets themaximum volume and the corresponding minimum (but not zero) inverse ornegative feedback.

In a preferred form of the invention, the siren input signal may be asine wave generated by a permanent magnet multiple-pole rotor movingrelative to, and in opposite polarity electromagnetic relationship withrespect to, a pair of similar output coil means whereby to cause similarelectrical signals of opposite phase to be produced therein by rotationof said rotor. These output coils feed the high-impedance input of themultiple-stage audio output amplifier means, which is biased to theclass C operating condition, and overdrive the input stage by somethingof the order of a twenty to one factor. This produces very fastswitching times in the square-wave output produced thereby and, thus,results in low transistor dissipation and high circuit efficiency.

In a preferred form of the invention, wail is produced through the useof a small DC motor which is mechanically coupled in drivingrelationship to the previously mentioned multiple-pole rotor which, inone specific exemplary form, may comprise a fifteen-pole generatoradapted to be driven by said DC motor to a maximum speed ofapproximately 4,000 rpm. This will, of course, produce fifteen times4,000 rpm. or 60,000 cycles per minute, which is 1,000 cycles persecond, thus causing a maximum frequency of approximately 1,000 cyclesper second to be produced and fed to the output loudspeaker means. Theoutput audio transformer is suitable for this frequency and theapparatus, when operated in the manner described hereinbefore, producesa large number of high harmonics, which appear at the speaker voice coiland which give the sound output a startling similarity to the soundproduced by a conventional mechanical air siren.

The small DC motor may be sequentially and alternately energized andde-energized by a siren activation switch operated manually or by anautomatic cycling means of any desired type, as mentioned hereinbeforeor as set forth with great particularity in one exemplary form in myhreinbefore-identified co-pending patent application.

When the siren activation switch energizing the siren driving motor isrepetitively energized and de-energized in the manner mentioned above,the rate of frequency rise is controlled by inertia of the rotatingportion of the motor and the multiple rotor of the sine wave generatorand the applied motor voltage.

In one preferred form of the invention, frequency fall rate is prolongedover that due to natural friction in the rotating system, by rectifyinga portion of the squarewave audio output from the output audio amplifiermeans and feeding a controlled amount of this DC voltage back to thesiren rotor drive motor in a positive feedback sense. As the speed ofrotation of said motor and said rotor reaches zero, said feedback signalalso drops to zero and consequently current to the motor becomescompletely out off and the motor and the multiple-pole rotor of thesiren generator come to a complete stop. This produces an output sirensound very similar to that produced by a conventional mechanical airsiren.

In one preferred form of the invention, the siren rotor driving motor isprovided with controllably operable motor braking means for positivelyand rapidly stopping rotation of the siren rotor upon operation of saidbraking means. In one preferred form, said braking means may comprise ashorting circuit and switch connected across a winding of the motor(although alternatively it may be similarly connected relative to themagnetic poles of the siren generator), which produces an inducedcurrent load and a consequent back emf as a result of rotation thereofwhereby to bring about a quick and positive stop thereof.

With the above points in mind, it is an object of the present inventionto provide novel apparatus having any of the advantages and/ or featuresreferred to herein, generically and/or specifically, and individually orin combination, and which is of relatively inexpensive, small,lightweight, foolproof construction adapted for ready mass manufactureat low cost whereby to be conducive to widespread use of the invention.

Further objects are implicit in the detailed description which followshereinafter (which is to be considered as exemplary of, but notspecifically limiting, the present invention), and said objects will beapparent to persons skilled in the art after a careful study of thedetailed description which follows hereinafter.

For the purpose of clarifying the nature of the present invention,several exemplary embodiments of the invention and/or various modifiedportions thereof, are illustrated in the hereinbelow-described figuresof the accompanying drawings and are illustrated in detail hereinafter.

FIG. 1 is an electrical schematic view of one exemplary combination formof the invention andillustrates the actual electrical circuit thereof,but does not show the real physical appearance and/or structure ofcertain of the physical components associated therewith. Also, variousportions of the complete combination shown in FIG. 1 are indicated bybeing enclosed in broken linesuch portions corresponding to certainidentifying expressions used throughout this application and being donefor purposes of convenience in clarifying what portions of the completecircuit means are meant by said identifying expressions as usedthroughout this application.

FIG. 2 is a fragmentary, electrical schematic view which I employ as anaid in illustrating the advantages of, and the operation of, the noveltwo-stage preamplifier means of the present invention as illustrated inthe complete combination form of the invention shown in FIG. 1. Itshould be clearly understood that this view does not illustrate thepreamplifier means of FIG. 1 but merely illustrates a preliminaryapproach thereto which is useful in understanding the advantages of thenovel preamplifier means of FIG. 1.

FIG. 3 is a view similar to FIG. 2, after a slight modification of thecircuit of FIG. 2 has been effected, which brings about a modificationof the gain thereof and which is also preliminary to the novelpreamplifier means of FIG. 1, but is not identical thereto.

FIG. 4 is a view similar to FIGS. 2 and 3 but illustrates a furthermodification of the electric circuitry of FIG. 3 which brings about aneffective division of the gain factor of the circuitry of FIG. 3 wherebyto produce two opposite'phase signals, each having a unity gain. Thisview also is preliminary to, but not identical to, the novelpreamplifier means of FIG. 1.

FIG. 5 is a view similar to FIGS. 2, 3, and 4 but illustrates a furtherslight modification of the arrangement illustrated in FIG. 4 whichfinally results in the novel preamplifier means of FIG. 1.

FIG. 6 is an electrical schematic view very similar to FIG. 1, butillustrates a slight modification thereof in two respects: first, as tothe preamplifier means, which is somewhat different in this modifiedform of the invention; and, second, as to the middle stage of thethreestage audio output amplifier means which, in this modified form ofthe invention, is an effectively doubled or parallel-connectedintermediate stage for the purpose of increasing the power-handlingcapacity thereof.

One exemplary combination form of the invention is illustrated inelectrical schematic form in 'FIG. 1, wherein it is shown as comprisinginput electrical signal-producing means coupled to an audio outputamplifier means which, in turn, is coupled to an audio outputtransformer means driving an output electroacoustic transducingloudspeaker means.

In the exemplary arrangement illustrated in FIG. 1, the above-mentionedinput electrical signal producing means comprises two different portionsof the apparatus as shown within the broken-line enclosures generallydesignated at 20 and 22. Broadly speaking, either one of said circuitportions designated at 20 and 22 may be said to comprise an inputelectrical signal producing means which is controllably operable forproducing a varying electrical input signal (an input signal withrespect to the succeeding portions of the apparatus) comprising twosimilar signal portions of opposite-phase relationship varying inaccordance with a desired sound output of the loudspeaker means and,when operative, in effective direct-coupled input relationship withrespect to the input side of the previously-mentioned audio outputamplifier means.

In the exemplary form of the invention illustrated in FIG. 1, the inputelectrical signal producing means portion indicated generally at 20comprises a siren signal generator means provided with controllablyoperable siren activation means comprising the siren switch 24, which isoperable when closed to effectively cause the siren signal generatormeans 20 to be in operative direct-coupled input signal producingrelationship with respect to the previously mentioned audio outputamplifier means which, in the exemplary form of the inventionillustrated, is designated generally by the reference numeral 26.

Since the audio output amplifier means 26 is coupled to the outputtransformer means, which is generally designated by the referencenumeral 2-8, and which, in turn, is coupled to the loudspeaker means,which is generally designated by the reference numeral 30, it will beunderstood that closure of the siren activation switch means 24 willresult in the broadcasting of a high-amplitude highenergy output soundfrom the loudspeaker means 30 of a siren-type sound corresponding to theoperation of the siren signal generator means 20.

The other input electrical signal producing means generally indicated at22 comprises a public address system input portion provided withcontrollably operable public address activation means, comprising apublic address switch 32 which is operable, when closed, to effectivelycause the public address system input portion 22 to effectively be inoperative direct-coupled input electrical signal producing relationshipwith respect to the previously mentioned audio output amplifier means 26so that a voice-operated input microphone 34 will cause said publicaddress input portion 32 to supply input to the audio output amplifiermeans 26, which will efiectively amplify same and feed same to theoutput audio transformer means 28, which will operate the loudspeakermeans 30 whereby to broadcast over a wide area the amplified initialvoice input to the microphone 34.

The foregoing brief description generally defines the exemplary form ofthe invention illustrated in electrical schematic form in FIG. 1. Moredetailed descriptions of the various elements thereof followhereinafter.

The previously mentioned siren signal generator means, generallyindicated at 20, comprises a small motor 36, which, in the exampleillustrated, is a DC motor, and which is adapted to be energizednormally, intermittently, and repetitively when the siren generatormeans 20 is intended to produce a conventional undulating siren sound ofthe kind normally employed by emergency vehicles as a warning signal, byintermittently closing and opening the previously mentioned sirenactivation switch 24, which may be done manually or by an automaticcycling means such as that shown in phantom at 38 in FIG. 1 andindicated as being coupled to said siren activation switch 24. Saidautomatic cycling means may be a clock-operated cycling switch, any ofvarious types of automatic cycling relays, various other functionalequivalents or it may be an automatic cycling means of the typedisclosed and claimed with greater particularity in my copending patentapplication Ser. No. 477,901, filed Aug. 6, 1965.

In any event, the siren activation switch 24 is adapted to berepetitively opened and closed, which will correspondingly repetitivelyconnect the storage battery 40 in series with the winding 42 of the DCmotor 36 whereby to cause the motor to be correspondingly repetitivelypower-rotated and de-energized.

The DC motor 36 is mechanically coupled in driving relationship withrespect to a multiple-pole permanent magnet rotor 44, which is mountedfor rotation by the motor 36 and is in oppositely directedelectromagnetically coupled relationship with respect to a pair ofsimilar output coil means 46, each of which is of the same impedance andresistance, and each of Which is coupled at the right end thereof, asindicated at 72, to a corresponding base element 48 of a correspondingtransistor 50 of the input pair of transistors of the previouslymentioned audio output amplifier means 26.

The above-described arrangement is such that the motor 36 may be adaptedto rotate the multiple-pole rotor 44 (which, in a preferred form,comprises a fifteen-pole rotor), to a maximum speed of 4,000 r.p.m. whenthe siren activation switch 24 is temporarily closed. Each magnetic poleof the multiple-pole rotor 44, when it passes the two coils 46, Willproduce similar sine waves therein, each of which is of equal magnitudeand opposite phase and will apply said equal, opposite phase, sine Wavesto the corresponding bases 48 of the corresponding input transistors 50of the audio output amplifier means 26 at the input terminals 72thereof.

Thus, it will be seen that for each revolution of the multiple-polerotor 44, each coil 46 will produce fifteen equal but oppositely phasedsine waves and that, since the rotor 44 is rotated at 4,000 r.p.m., thiswill result in a frequency of 60,000 cycles per minute or 1,000 cyclesper second being produced in each of said coils 46. This maximum 1,000cycles per second input signal to the audio output transformer means 26will result in a greatly amplified and extremely loud siren-type outputsignal from the loudspeaker means 30 of a corresponding maximumfrequency of approximately 1,000 cycles per second, which is correct forthe type of siren output sound desired. The output audio transformermeans 28 is designed to handle this frequency effectively, and a largenumber of high harmonics will be found to appear at the loudspeakervoice coil 52, thus giving the output siren sound a startling similraityto the sound produced by a conventional mechanical air siren.

The rate of frequency rise of the siren generator 20 is controlled bythe inertia of the rotor 44 and the voltage applied to the winding 42 ofthe motor 36.

The frequency fall rate of the siren generator 20 may be prolonged overthat due to natural friction in the system in order to more closelysimulate the output sound of a conventional mechanical air siren, byrectifying the square wave audio output of the audio output amplifiermeans 26 and feeding a controlled amount of this rectified DC voltageback to the winding 42 of the siren driving motor 36. This isaccomplished by the two circuit leads 54 and the two half-waverectifiers 56 which are connected together and are fed through theresistance 58 to the terminal 60 at one end of the winding 42 of themotor 36.

The elements just referred to above may be said to effectively comprisea siren wail cutoff retarding means comprising positive feedback meanscoupled to the audio output amplifier means 26 and feeding back afeedback signal proportionate to the output thereof to the siren rotordriving motor 36 in a direction such as to extend the period of rotationthereof after the siren activation switch 24 has been opened. Thiscauses the cutoff end of the undulating siren sound to simulate thatproduced by a conventional mechanical air siren. It should be understoodthat as the speed of rotation of the rotor 44 and the motor 36 reacheszero, so does the above-mentioned feedback signal and, therefore,current through the winding 42 of the motor 36 becomes completely cutoffand the siren generator 20 becomes completely inactivated untilsubsequent closure of the siren activation switch 24 again occurs.

In some cases, it may be desirable to be able to stop the sirengenerator 20 very quickly and, where such is desired, an optional motorbraking means may be provided for positively and extremely rapidlystopping the rotation of the siren rotor 44 and the motor 36. Oneexemplary form of such a motor braking means is indicated at 62 where itmerely comprises a shorting circuit 64 and a normally open shortingswitch 66 connected across the winding 42 of the motor 36 in a mannersuch that temporary closure of the normally open shorting switch 66 willeffectively short-circuit the motor winding 42 so that it effectivelysees an induced current load and a consequent back emf as a result ofrotation of the motor 36 and the rotor 44 and thus causes same to cometo a quick stop. However, it should be noted that the braking means 62is an optional feature and may be eliminated entirely in certain formsof the invention. This is also true of the previously mentioned sirenWail cutoff retarding means for prolonging the frequency fall rate ofthe siren output signal, as described in detail hereinbefore.

The audio output amplifier means 26, in the exemplary first form of theinvention illustrated, comprises a threestage, direct-coupled, push-pullamplifier, which is operated as a class C amplifier when the sirenactivation control switch 24 is closed, but which acts as a class Bamplifier when the previously mentioned public address system activationswitch 32 is closed.

The above-mentioned three-stage audio output amplifier 26 comprises thefirst or input stage shown at the left thereof in FIG. 1 (which includesthe previously mentioned pair of transistors 50) connected in inputrelationship with respect to the second or intermediate stage of saidaudio output amplifier means 26 (which includes a second pair oftransistors generally indicated at 68) connected in input relationshipwith respect to the third or last stage of said audio output amplifiermeans 26 (which includes a third pair of transistor means generallyindicated at 70).

Each of the three stages just defined of the audio output amplifiermeans 26 and the three pairs of transistors 48, 68, and 70 thereof areconnected in effective push-pull relationship with respect to the inputterminals 72 thereof and with respect to the output terminals 74thereof, which, in turn, are connected to the transformer winding 76 ofthe audio output transformer rneans generally indicated at 28, inpush-pull relationship whereby to provide a high-amplitude, high-poweroutput audio signal having a minimum of distortion and coupled to thepreviously mentioned loudspeaker voice coil 52 for producing a very highvolume output sound and for doing so with great efiiciency. The systemjust described may readily produce an output ranging from watts to wattspeak output, which is entirely adequate for the purposes of the presentinvention.

The amount of drive at the input terminals 72 of the audio outputamplifier means 26 is normally high enough to maintain saturation downto about 200 cycles per second. Below this frequency, the square wavebegins to lose sharpness of the peaks and the transition time becomesprogressively longer. As zero frequency is approached, the output of thesiren generator means 2t] falls off rapidly, thus preventing excessivecurrents from damaging the loudspeaker means 30 or any of the varioustransistors because of the lowering of the reactance of the transformermeans 28 at such lower frequencies.

The other input electrical signal producing means, generally designatedat 22, and comprising the previously mentioned public address systeminput portion, includes the previously mentioned input microphone means34, which, broadly speaking, may comprise any type of acoustic-electrictransducing means which is coupled in input relationship with respect tothe two-stage, directcoupled, phase-splitting input audio signalpreamplifier means, which comprises that portion of the circuit shown inFIG. 1 contained in the broken-line enclosure generally designated bythe reference numeral 22 and exclusive of the microphone 34. Saidpreamplifier means will be generally designated by the reference numeral78 hereinafter, and it should be understood as comprising the elementsjust mentioned.

Said preamplifier means 78, in the first exemplary form of the inventionillustrated, includes a first transistor 80 and a second transistor 82,each comprising one of the stages of said two-stage preamplifier meanswhich are connected in complementary, direct-coupled, phase-splittingrelationship so that two equal signals of opposite phase will be appliedtherefrom and through the associated circuit elements to the inputterminals 72 of the previously mentioned audio output transformer means26. These equal and opposite phase signals are of the proper voltage(usually of the order of /2 volt) to properly bias the input transistors50 of the audio output amplifier means 26 for class B operation insteadof their previous class C operation, when the siren activation switch 24was closed. This biasing into a class B mode of operation occurs whenthe public address activation switch 32 is closed.

The novel preamplifier means 78 includes novel gainmodifying resistancemeans 84 and 86 and gain-modifying variable resistance means 88 and 90taking the form of a pair of controllably adjustable coupled or gangedvariable potentiometers coupled in a controllably variable negativefeedback manner with respect to the preamplifier means 78 such as toeffectively define a DC balanced negative feedback circuit withoutfrequency discrimination which is operable for controlling the volume ofoutput sound produced by the loudspeaker means 30 as an inverse functionof said negative feedback. Therefore, it'will be understood that coupledoperation of the two potentiometers 88 and 90 by a volume-adjustingknob, or the like, (not shown since such knobs are well-known in theart), in a direction such as to increase the negative feedback willreduce the volume of output sound produced by the loudspeaker means 30,while operation thereof in a direction such as to decrease the negativefeedback will increase the volume of the output sound produced by theloudspeaker means 30. Appropriate mechanical stops such as are indicateddiagrammatically at 92 (although a lesser number than shown on FIG. 1may actually be required in actual practice) act to limit the adjustmentof the coupled potentiometers 88 and 90 so that the maximum volumesetting, corresponding to minimum (but not zero) negative feedback, willbe automatically determined, as will the corresponding oppositecondition with minimum volume and maximum negative feedback. However,this latter stop may be eliminated, if desired.

FIGS. 2 through 5 illustrate in successively modified steps theoperation of the novel preamplifier means 78 of FI 1 (and, also, of FIG.5, which is substantially equivalent thereto), and it should beunderstood that the showings of FIGS. 24 are merely illustrative and arenot the novel preamplifier means 78 of the present invention. Thesefigures will now be described in sequence, and

parts thereof which correspond to those of the novel preamplifier means78 of FIG. 1 are designated by the same reference numerals. However, inthe case of parts which have been modified from the showings of any ofFIGS. 25 to their final showings in the novel preamplifier means 78 ofFIG. 1, said parts are indicated by similar reference numerals, which,however, are primed, and double primed, in the case of resistors 86",98", and 100" in FIG. 2, in accordance with the number of themodifications thereof.

FIG. 2 shows a conventional phase-splitting circuit using twotransistors 80 and 82 in a complementary connection. These act incombination similar to a single PNP transistor having a beta equal toapproximately the product of the separate betas. This type ofarrangement produces an in-phase output signal at the terminal 94 and anout-of-phase output signal at the terminals 96. Both output signals haveunity gain. Bias resistors 98 and 100" are chosen with values such as toallow maximum swing of the output voltages. The two output signals at 94and 96 are at DC levels of 1.5 volts and -4.5 volts, respectively, andnormally would have to be capacitycoupled to any succeeding or followingpush-pull amplifier.

FIG. 3 shows a modification of the value of the resis tor 86" of FIG. 2(which, because of said modification, is indicated by the referencenumeral 86') and comprising an effective multiplication thereof by afactor of six and, in the arrangement illustrated, a constant gain ofsix is produced in the out-ofphase output signal at the terminal 96,said gain being practically independent of the transistorcharacteristics. Voltage swing in this case is limited to approximately0.5 volt input and 3 volts output. It should be noted that, in thiscase, the out-of-phase output signal at the terminal 96 has a value of 3volts while the inphase output signal at the terminal 94- has a value of0.5 volt. Since the voltage swing determined by the modified resistors98 and 100 is limited to 0.5 volt input, this means that the in-phaseoutput signal at the terminal 94 is only said 0.5 volt (thus having again of unity) and that the out-of-phase output signal at the terminal96 has said voltage of 3 volts (based upon a gain of six).

FIG. 4 divides the out-of-phase signal at the terminal 96 by a factor ofsix so that the effective gain thereof is again equal to unity andcomprises -0.5 volt at the terminal 1M while the in-phase output signalat the terminal 94 remains O.5 volt. In other words, the resistances 84and 88 produce the effective division or modification of gain of theout-of-phase output signal so that the inhase output signal at 94 andthe out-of-phase output signal at 104 are of equal amplitude (eachhaving a gain of unity) and are of opposite phase. In other words, theDC levels at each of said terminals 94 and 104 is of the proper valuefor biasing an input transistor of a push-pull amplifier for class Boperation and, therefore, the output signals at the terminals 94 and 104can be directly coupled to such a push-pull amplifier and do not need tobe capacity-coupled thereto in the manner of the circuit arrangementillustrated in FIG. 2, as previously described. In other words, withrespect to FIG. 4, it should be noted that when the gain factor isnumerically equal to the supply voltage, the bias will always be -0.5 atbalance (balance being controlled by the resistors 98' and 100, aspreviously mentioned).

FIG. 5 illustrates the replacement of the two resistors and 88' of FIG.4 by a corresponding pair of coupled or ganged potentiometers 88 and 90of linear taper and the addition of resistor 109. Adjustment of saidpotentiometers 88 and 90 in a manner effecting a decrease in negativefeedback will cause a corresponding increase in gain and, therefore, involume of the sound produced by the loudspeaker means 30 of FIG. 1.Conversely, adjustment of the ganged potentiometers 88 and 90 of FIG. 5and of FIG. 1 in the opposite direction for increasing the negativefeedback, will cause a corresponding decrease in volume of output sound.DC voltages throughout the preamplifier circuit means 78 of FIGS. 1 andremain essentially constant and all frequencies are treated alike up tothe limitations of the transistors employed.

It should be noted that the balance resistors 98' and 100 of FIG. 4 havealso been modified and effectively combined into a corresponding balancepotentiometer 108 including a corresponding two resistance portions 98and 1&0 clearly illustrated in both FIG. 5 and FIG. 1.

For the purpose of enhancing the disclosure, a partial specification ofcertain of the circuit elements or components employed in the exemplaryfirst form of the invention illustrated in FIG. 1 are listedhereinbelow. The values listed are to be considered as exemplary of oneform of the invention which has been built and which is fully operativebut are not to be construed as specifically limiting the invention tothe particular specification set forth below as follows:

Potentiometer 108 ohms 500,000 Potentiometer 88 do 1,000 Potentiometer90 do 50 Resistor 84 do 5,600- Resistor 86 do 330 Each coil 46 do 300Resistor 109 do 1,000 Each resistor 110 do 220 Each resistor 112 (3watts) do 25 Resistor 114 do 330 Resistor 58 do 220 Transistor 80 andthe pair of transistors 50 2N3638 Transistor 82 and the pair oftransistors 68 2N3405 Pair of transistors 70 2N3372 Battery 40 volts 12The winding 76 of the audio output transformer 28 includes twenty turnsin the portion between the terminal 74 on each side of the center tap116 connected to the positive terminal of the battery 40 and eight turnsin the winding portion positioned outwardly thereof between the terminal74 and the terminal 118 on each side of the transformer winding. Thisprovides two pairs of output terminals 120 and 118, respectively,adapted for connection to an 8-ohm voice coil 52, a l6'ohm voice coil,any other type of voice coil, or to multiple such voice coils ofvirtually any type.

In addition to the above, resistor 86" in FIG. 2 is 50 ohms, and whenmodified to comprise resistor 86 in FIGS. 3, 4, and 5 is 300 ohms. Theresistor 90 in FIGS. 2-4 is 50 ohms prior to its modification into thepotentiometer 90 of FIGS. 5 and 1 where it comprises a SO-Ohmpotentiometer. The resistor 88' of FIG. 4 has a value of 1,000 ohmsprior to its modification, as shown in FIGS. 5 and 1, into the 1,000-ohmpotentiometer 88.

FIG. 6 illustrates a modified form of the invention very similar to thefirst form of the invention illustrated in FIG. 1, with certain portionsthereof modified to some extent and corresponding parts are designatedby corresponding reference numerals, followed by the letter a, however.

It will be noted that in the FIG. 6 modification of the invention thepreamplifier means 78a is somewhat different from the preamplifier means78 of the first form of the invention and the two transistors 80a and82a thereof are no longer in the complementary connection illustrated inthe first form of the invention, but, rather, are connected as adifferential amplifier in which the out-ofphase signal is produced intransistor 82a by means of the coupling effected by the common emitterresistor 122. This configuration allows the input connection of themicrophone 34a to have one side at effectively ground potential. Otheradvantages are so-called common mode rejection, a term well-known in theart, and the provision for symmetry in the negative feedback circuits.Proper choice of resistance values and replacement of the usualcollector load resistors by the siren generator coils 46a produces twoequal and oppositely phased output signals at the input terminals 720 ofthe two input transistors 50a of the audio output amplifier means 26::for subsequent operation of the remainder of the circuit in a mannervery similar to that described hereinbefore in connection with the firstform of the invention illustrated in FIG. 1, although the gangedpotentiometers 88a and 90a are somewhat differently arranged than in thefirst form of the invention and are embodied in the audio outputamplifier means 26a rather than in the preamplifier means 780, as in thefirst form of the invention. Also, in the FIG. 6 modification of theinvention, a second pair of transistors 68a is effectively connected inparallel with respect to the first pair of transistor means 68a in thesecond stage of the audio output amplifier means 26a. However, this ismerely to increase the power-handling capacity of said second stage andeach such doubled pair of transistors might have a single transistorcapable of handling a larger power substituted in lieu thereof. Also, itshould be noted that, if desired, the second stage transistors 68 of thefirst form of the invention of FIG. 1 may be effectively doubled andparallel connected in the manner of the transistors illustrated at 68ain the FIG. 6 modification of the invention. Otherwise, the FIG. 6modification of the invention is very similar to, and functions in amanner substantially equivalent to, the first form of the inventionillustrated in FIG. 1, although the novel preamplifier means 78 of thefirst form of the invention illustrated in FIG. 1 has a number ofadvantages over the modified preamplifier means 78a of the modified FIG.6 form of the invention for various of the reasons noted at variouslocations hereinbefore.

It should be noted that while a single loudspeaker means 30 is shown inthe first form of the invention in FIG. 1 and is shown at 30a in theFIG. 6 modification of the invention, actually one or more loudspeakersmay be employed and multiple-choice output terminal connections of thetype illustrated in FIG. 1 may be employed for connecting one or moresuch loudspeakers or for connecting loudspeakers having voice coils ofdifferent values of impedance or resistance.

Also, it should be noted that any of the forms of the invention may bepowered by suitable transformer and rectifier power supply means in lieuof the storage battery 40 of FIG. 1 and 40a of FIG. 6 so that anyconventional source of alternating current of any available voltage maybe transformed into a desired voltage value and appropriately rectified(and, if desired, filtered) to comprise a functional equivalent to thestorage battery 40 or 40a and, since such arrangements are well-known inthe art, they are not illustrated.

Additionally, it should be noted that a conventional tradio of either anAM or FM type, such as is found in many emergency vehicles may, ifdesired, be coupled to either the audio output amplifier means 26 ofFIG. 1 (or 26a of FIG. 6) and provided with controllably operable switchmeans for operatively coupling same together, or provided with barriermeans operative for the same coupling purposes when the audio outputsignal amplitude of such a radio exceeds a predetermined value, so thatunder such conditions the loudspeaker means 30 of FIG. 1 or 30a of FIG.6) will act as an output speaker for the radio and will broadcast thesignal therefrom. Also, if desired, the audio output signal from such aconventional radio may be coupled directly into the audio outputtransformer 28 and/or loudspeaker means 30 of FIG. 1 or thecorresponding elements 28a and/ or 30:: of FIG. 6, if desired, for thesame purpose.

I claim:

1. An audio amplifying and transducing apparatus for producing andemitting a high-amplitude, high-energy sound output, comprising: amultiple-stage direct-coupled push-pull audio output amplifier meanshaving multiple audio output amplifier stages in sequentialdirect-coupled relationship with each audio output amplifier stageincluding a pair of power amplifying transistors; an audio outputtransformer means coupled in push-pull relationship with respect to theoutput side of said audio output amplifier means; an outputelectro-acoustic transducing loudspeaker means coupled to the output ofsaid audio output transformer means; and input electrical signalproducing means controllably operable for producing a varying electricalinput signal comprising two similar signal portions of opposite phaserelationship varying in accordance with a desired sound output andeffectively in directcoupled input relationship with respect to theinput side of said audio output amplifier means, said input electricalsignal producing means comprising a siren signal generator mean providedwith controllably operable siren activation control means foreffectively causing said siren signal generator means to be in operativedirect-coupled input relationship with respect to said audio outputamplifier means and for causing said audio output amplifier means to bebiased for class C operation said siren signal generator meanscomprising a motor provided with and driving a multiple pole magneticrotor means provided with and in oppositely directed electromagneticallycoupled relationship with respect to a pair of similar output coil meansadapted to have similar electric signals of opposite phase producedtherein by rotation of said rotor means.

2. Apparatus as defined in claim 1, wherein said siren rotor meansdriving motor is provided with controllable operable braking means forpositively stopping rotation of said siren rotor means upon operation ofsaid braking means and comprising a shorting circuit and switchconnected across a winding of said motor which effectively produces aninduced current load and a consequent back emf as a result of rotationthereof whereby to bring about a quick stop thereof.

3. Apparatus as defined in claim 1, wherein said siren signal generatormeans is adapted to be intermittently and repetitively activated andinactivated by said controllably operable siren activation control meansand is provided with siren -wail cutoff retarding means operable forpreventing abrupt cutoff of output sound from said loudspeaker meanswhen said siren signal generator means is inactivated by de-energizationof said motor driving said rotor.

4. Apparatus as defined in claim 3, wherein said siren wail cutoffretarding means comprises positive feedback means coupled to the outputof said audio output amplifier means and including rectifier means forrectifying same and feeding back a signal proportionate thereto to saidsiren rotor driving motor in a direction such as to extend the period ofrotation thereof after inactivating operation of said siren activationcontrol means, thus preventing abrupt cutoff of output sound from saidloudspeaker means.

5. Apparatus as defined in claim 1, wherein said input electrical signalproducing means also comprises a public address system input portionprovided with controllably operable public address activation controlmeans for effectively causing said public address system input portionto be in operative direct-coupled input relationship with respect tosaid audio output amplifier means and for causing said audio outputamplifier means to be biased for class B operation.

6. Apparatus as defined in claim 5, wherein said public address systeminput portion comprises an input acoustoelectric transducing meansprovided with and coupled in input relationship with respect to amultiple-stage directcoupled phase-splitting input audio signalpreamplifier means.

7. Apparatus as defined in claim 5, wherein said public address systeminput portion comprises an input acoustoelectric transducing meansprovided with and coupled in input relationship with respect to atwo-stage directcoupled phase-splitting input audio signal preamplifiermeans having two input audio signal preamplifier stages in complementarydirect-coupled phase-splitting relationship, with each of the two inputaudio signal preamplifier stages including a power amplifying transistorconnected in effective complementary relationship with respect to thetransistor of the other input audio signal preamplifier stage.

8. Apparatus as defined in claim 7, wherein said input audio signalpreamplifier means has output circuit means connected thereto and isprovided with gain-modifying variable resistance means operable to causethe no-inputsignal voltage outputs of said preamplifier meansdirectcoupled to said audio output amplifier means to be of equalmagnitudes optimum for class B operation thereof.

9. Apparatus as defined in claim 8, wherein said gainmodifying variableresistance means includes a pair of controllably adjustable coupledvariable potentiometers coupled in a controllably variable negativefeedback manner with respect to said preamplifier means and effectivelydefining a DC balanced negative feedback circuit without frequencydiscrimination operable for controlling the volume of output sound as aninverse function of negative feedback.

References Cited UNITED STATES PATENTS 792,103 6/1905 Winter et al.318375 2,920,189 1/1960 Holmes 33017 X 3,051,944 8/1962 Smith 3403843,096,487 7/1963 Lee 33017 JOHN W. CALDWELL, Primary Examiner.

CHARLES M. MARMELSTEIN, Assistant Examiner.

U.S. Cl. X.R. 33017; 340405

